1. Field of the Invention
The present invention relates to plasma systems in which metal particles are sprayed by a plasma stream, and more particularly to a method of removing oxides from metallic powder particles introduced into a plasma stream.
2. History of the Prior Art
Refractory materials such as titanium and tantalum and even aluminum are difficult to produce in powdered form without an oxide layer being present on the surface of the powder particles. A typical process of forming the powder involves melting the metal and then introducing the molten metal into a gas stream. As the powder particles are formed, the highly oxidizable nature of the material causes an oxide layer to form on the outside of the particles. Such oxidation can be minimized by using other processes to form the powder, but such processes tend to be relatively expensive.
A further problem arises when metallic powders, particularly those of a highly oxidizable nature, are to be coated on a substrate using a plasma stream. Even where the powder is produced in a relatively oxide-free form, the mere process of introducing the powder into a plasma stream for coating on the substrate typically results in some oxidation of the powder particles. This is particularly true of highly oxidizable materials such as titanium, tantalum and aluminum. Workers skilled in the art have observed the tendency of most metallic particles to undergo some oxidation as they are sprayed in a plasma stream. A natural reaction to this has been a desire to spray the powdered materials quickly so that they are deposited on the substrate before substantial oxidation occurs. However, the speed of the oxidation process has been difficult to determine. Moreover, even where supersonic speeds of the plasma steam are produced such as through the use of a vacuum source to provide low static pressure, some oxidation of the metallic particles is still observed.
An example of a conventional supersonic plasma system is provided by U.S. Pat. No. 4,328,257 of Muehlberger et al. which issued May 4, 1982 and which is commonly assigned with the present application. The Muehlberger et al. patent describes a plasma system in which a vacuum source creates a low static pressure within an enclosure containing a plasma gun and a workpiece located downstream of the plasma gun. The plasma gun ionizes an inert gas to produce a plasma stream. The plasma stream flows from the plasma gun to the workpiece at supersonic speeds in the presence of the low static pressure provided by the vacuum source. Metallic powder introduced into the plasma stream at a location adjacent the plasma gun is carried to the workpiece where it is deposited on the workpiece as a coating.
The plasma system described in the Muehlberger et al. patent employs switchable transfer arc power supplies which are advantageously employed to initially establish a negative or cathodic condition at the workpiece for purposes of cleaning the workpiece. Thereafter, the workpiece is made positive relative to the plasma gun to enhance the depositing of the metallic powders introduced into the plasma stream onto the workpiece. In spite of the supersonic speeds of the plasma stream, it has been observed that some oxidation of the metallic powder still occurs as it travels along the plasma stream. This is especially true in the case of the highly oxidizable refractory materials, even when such materials are introduced into the plasma stream in a relatively pure, oxide-free form. As previously noted such materials are difficult to produce in powdered form without the formation of an oxide coating on the particles, with the result that the less expensive processes for producing the metallic powders provide the powder particles with an oxide coating before they are even introduced into the plasma stream. All of this results in the presence of substantial oxides in the coating formed on the workpiece.
An alternative approach to the spraying of metallic powders in a plasma stream which minimizes oxides is described in U.S. Pat. No. 4,689,468 of Muehlberger which issued Aug. 25, 1987, and which is commonly assigned with the present application. In the '468 patent of Muehlberger, a main plasma gun and a second or clean-up plasma gun simultaneously provide transfer arcs of opposite polarities at a common workpiece or substrate, with the result that oxides at the workpiece or substrate are significantly reduced. However, such oxide reduction comes at the expense of a more elaborate system requiring the presence of the second plasma gun and a separate set of power supplies therefor.
Accordingly, it would be advantageous to provide a process for removing the oxide coatings from highly oxidizable metal particles using a single plasma gun. It would furthermore be advantageous to provide a process for removing the oxide coatings from metal particles in conjunction with the spraying of such particles onto a workpiece or substrate.